The present invention relates to a malleable iron having high hardness and good lubricity and wear resistance, and more particularly, this invention relates to a malleable iron having at least 250 graphite nodules per square millimeter and a method of making such a metal.
There are three types of cast irons: malleable, ductile and gray iron. Of these, malleable and ductile irons can be plastically deformed. These irons can be differentiated by their microstructures. Gray iron has most of its carbon in the form of flakes which resemble the shape of potato chips. Malleable iron has most of its carbon in the form of irregularly shaped graphite nodules also known as "temper carbon" which resemble the shape of popped popcorn. Ductile iron, which can also be referred to as "nodular" or "spheroidal" iron, contains carbon in the form of small round graphite spherulites.
The carbon in malleable iron is predominantly in the form of graphite. Typically, malleable iron contains about 50 to 100 graphite nodules per mm.sup.2.
Malleable iron is first cast as a white iron and then annealed at temperatures that result in the decomposition of cementite (iron carbide, Fe.sub.3 C) and convert the iron matrix into ferrite, pearlite, or combinations thereof. Ferrite is practically pure iron. Pearlite is a eutectoid structure comprised of alternative layers of ferrite and cementite. The chemical composition of malleable iron is generally 2.0 to 2.9% carbon, 0.9 to 1.9% silicon, 0.2 to 1.0% manganese, 0.02 to 0.2% sulfur, and 0.02 to 0.2% phosphorus. Unless otherwise noted, all percentages herein are by weight. Small amounts of chromium, boron, copper, nickel and molybdenum may also be present.
The iron for most present-day malleable iron is melted in coreless induction furnaces. The melting can be accomplished by batch cold melting or by duplexing. Molds are produced in green sand, silicate CO.sub.2 bonded sand or resin-bonded sand (shell molds). Then the melted iron is poured into the molds. Molten iron produced under properly controlled melting conditions solidifies with all carbon in the combined form, producing white iron for ferritic or pearlitic malleable iron. After the casting solidifies and cools, the metal is in a white iron state and any gates, sprues and feeders are removed from the castings. The castings are then heat treated. It is known to add agents such as magnesium, cerium, boron, aluminum and titanium to the molten metal to enhance the nodular forming properties.
The initial annealing converts the carbon that exists in combined form massive carbides (Fe.sub.3 C) or microconstituents in pearlite into temper carbon. Conventionally, the first state anneal is approximately 9-15 hours and up to 5 days at about 900 to 970.degree. C. (1650 to 1780.degree. F.). However, irons with lower silicon contents may require as much as 20 hours for completion of first-stage annealing. The initial anneal is followed by additional heat treatments that produce the desired matrix microstructures in the iron.
Conventionally, such a method produces a nodule count of about 50 to 100 discrete graphite particles per square millimeter as measured in a photomicrograph magnified at 100.times. (hereinafter all references to nodules/mm.sup.2 are assumed to be measurement in a photomicrograph at 100.times.). The particle distribution is random, with short distances between the graphite particles. Temper carbon is formed predominantly at the interface between primary carbide and saturated austentite at the first stage annealing temperature, with growth around the nuclei taking place by a reaction involving diffusion and carbide decomposition.
Conventional malleable iron has fewer nodules (50 to 100 nodules/mm.sup.2). Parts made from these irons do not exhibit sufficient lubricity for many applications requiring high wear. The diameter of the graphite nodules is large and abrasion tends to lift the nodules up causing them to pop out and form craters. This causes the machine parts to seize up and the parts fail. Thus, there is a need for a malleable iron which has an increased number of graphite nodules and a method of making such a metal.